There are many forms of hovering aircraft that have been conceived over the past 130 years, starting with Alphonse Penaud's planophore and moving forward. Nearly all of these aircraft have taken the form of what we have come to know as “conventional” helicopters, that is, an aircraft with one or more main rotors and possibly a tail rotor. The fundamental problem with all of these conventional helicopters is that once they start to move forward in horizontal flight, the mostly horizontal rotor experiences extreme aerodynamic difficulties. Chief among those are adverse transonic effects on drag on the advancing blades and unsteady stall on the retreating side.
Several attempts have been made to skirt these high speed effects with limited success through the years. These include pitching the rotors over so that oncoming air is ingested primarily by rotors which are oriented normal to oncoming flow. The XV-15 and V-22 both use this general scheme for achieving higher speed flight, but are also hamstrung in high speed flight as transonic rotor effects eventually creep in and retard forward flight performance in the mid-subsonic range.
Another approach to achieving hovering flight and high speed dash speeds was seen in the various body pitch aircraft. Several early aircraft which “converted” from hovering to airplane mode flight included the Convair XFY-1 and the Lockheed XFV-1. These aircraft used counterrotating propellers located at the front of the aircraft to achieve vertical take-off and landing while an empennage assembly provided nearly all flight control on takeoff and landing. Because of extremely high disk loading and very low control authority, this scheme was shown to be difficult to manage at best. Accordingly, it was abandoned nearly 40 years ago.
Modern convertible aircraft include have taken advantage of aerodynamic effects associated with ring-wing flight. The successful XQ-138 described in U.S. Pat. No. 6,502,787 Convertible vertical take-off and landing miniature aerial vehicle employed a ducted fan for vertical take-off, landing and hovering mode flight. This aircraft enjoyed extended hover times coupled with comparatively high speed dash speeds, again, only through the mid-subsonic flight regimes because of transonic effects.
Several other ducted fan aircraft have been robustly researched recently including those described in U.S. Pat. No. 5,295,643, Unmanned Vertical Take-off and Landing, Horizontal Cruise, Air Vehicle, U.S. Pat. No. 6,691,949 Vertical Take-Off and Landing Aerial Vehicle, U.S. Pat. No. 7,032,861, Quiet Vertical Takeoff and Landing Aircraft, U.S. Pat. No. 7,681,832 Ducted Fan Air Vehicle with Deployable Wings and U.S. Pat. No. 7,658,346 Double Ducted Hovering Air Vehicle, US Pat. Application 20060049304, Quiet Vertical Takeoff and Landing Aircraft Using Ducted, Magnetic Induction Air-Impeller Rotors. These and many others employ a ducted fan configuration which can help increase rotor efficiency and offers extra volume for mission packages and fuel storage, but it comes at a high price. That price is seen most vividly as higher airspeeds are sought.
Because a ducted fan assembly typically possesses an extremely high wetted area with respect to the clean fuselage/rotor combination, its profile and parasite drag components become high as flight speeds are increased. Indeed, the drag components are so profound that no ducted fan aircraft capable of sustained hovering flight has ever been able to sustain flight beyond mid-subsonic flight speeds.
There are a host of other aircraft concepts which are related to hovering aircraft and uninhabited aerial vehicles (UAVs), but are in support of these older, aforementioned concepts. Specifically, many of these designs are fundamentally not compatible with aircraft that are capable of sustained supersonic flight: US Pat. Application no. 20070262195, UAV With Control and Stability System, US Pat. Application no. 20070244608, Ground control station for UAV, US Pat. Application no. 20070200027, Aerial robot, US Pat. Application no. 20070129855, Device and Method of Automated Construction of Emergency Flight Path for Aircraft, US Pat. Application no. 20070069083, Self-Contained Avionics Sensing And Flight Control System For Small Unmanned Aerial Vehicle, US Pat. Application no. 20070051848, Landing gear for a hovercraft, US Pat. Application no. 20070034738, Aerodynamically Stable VTOL Aircraft, US Pat. 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